编写程序实现字符串的基本操作，包括求串长、赋值、比较、串连接、求子串、定位和替换算法。</span>

以下是Python语言的实现：

1. 求串长

def strlen(s):
    return len(s)

2. 赋值

def strassign(s):
    return s

3. 比较

def strcmp(s1, s2):
    if s1 == s2:
        return 0
    elif s1 < s2:
        return -1
    else:
        return 1

4. 串连接

def strcat(s1, s2):
    return s1 + s2

5. 求子串

def substr(s, pos, length):
    return s[pos:pos+length]

6. 定位

def strpos(s, target):
    return s.find(target)

7. 替换

def strreplace(s, old, new):
    return s.replace(old, new)

示例：

s = "hello world"
print(strlen(s))  # 11

s1 = strassign(s)
print(s1)  # hello world

s2 = "world"
print(strcmp(s1, s2))  # 1

s3 = " python"
print(strcat(s1, s3))  # hello world python

print(substr(s1, 0, 5))  # hello

print(strpos(s1, "world"))  # 6

print(strreplace(s1, "world", "python"))  # hello python